1. Field of the Invention
This invention relates to a method and apparatus for measuring the flow rate of a fluid and, more particularly, to an apparatus and method for measuring fluid flow rate in a vessel by monitoring the movement of ions between at least one of a plurality of conductors and the fluid.
2. Description of Related Art
Diagnostic catheters have heretofore been provided measuring intravascular pressure and blood flow using thermal dilution, dye dilution and oxygen consumption methods. Intravascular catheters have also been developed which measure instantaneous flow velocity utilizing ultrasonic Doppler transducers to measure the "Doppler shift" created by movement of red blood cells, acting as targets, with the blood vessel or organ to which a measurement is being made. Doppler systems of the past typically used Doppler catheters which were only useful for the measurement of flow velocity within a small sample volume contained within the blood vessel of interest.
Other methods, systems and apparatuses have been designed to measure flow rate using ultrasonic techniques. For example, U.S. Pat. No. 5,174,295 discloses a system for measuring characteristic of flow of liquid in a vessel of a patient comprising a transducer positioned in a vessel in a patient of supplying ultrasonic energy. The transducer produced a substantially uniform beam which encompassed the vessel. The transducer received ultrasonic energy back scattered from the plasma and red blood cells and provides an electrical output signal which was detected by a detector which generated a moment signal in response thereto. The signal was then normalized to provide an output characteristic of the flow of the liquid in the vessel.
A disadvantage of the ultrasonic systems of the past was that they required relatively high driving voltages. Another disadvantage is that they were often complex in design and it was sometimes difficult to maintain stability. For example, in ultrasonic systems which use coaxial cable, the coaxial cable is possessed of capacitance. In general the smaller the overall diameter of the cable, the higher its capacitance. Cable capacitance tends to attenuate high frequency AC signals. As cable capacitance increases, the attenuation increases and this attenuation can become significant as cable diameters become smaller and smaller, such as would be required for use with a catheter. This attenuation is undesirable. In addition, these systems typically used a cable having a fairly large capacitance. The impedance and capacitance often adversely affected the signal-to-noise ratios which, in turn, affected the ultimate flow reading.
As mentioned above, thermal dilution devices have also been used to measure fluid flow rates. In general, these devices provided a thermal conductor, such as a thermistor, which was energized with a current to cause the thermal conductor to be set at a desired temperature. As fluid flowed past the thermistor, heat was dissipated by the fluid. Additional current was supplied to the thermistor to maintain the temperature of the thermistor at a predetermined level. The change in additional current was then measured, thereby providing an indication of the flow rate of the fluid. To maintain the current levels needed, the thermal dilution device often required voltage input levels of, for example, 1.5 volts at 30 milliamps for 0.045 watts heating at the thermistor site.
A problem with the thermal dilution devices of the past is that they tended to be slow in measuring flow rate. In addition, sometimes their accuracy depended in large part on the mass of the thermal conductor. Finally, they also typically required relatively high current levels to cause the temperature of the thermal conductors to be maintained at the desired temperature.
Still another problem with flow measuring instruments of the past is that they were not typically multifunctional in that they were not typically designed with other sensing apparatus. For example, they did not include an optical sensor for visual inspection in conjunction with flow measuring. Also, they were not typically used with balloon catheters of the type used in heart catheterization procedures.
What is needed, therefore, is a measuring device, method and system which overcomes the problems of the prior art and which provides an accurate, yet relatively inexpensive way to measure the flow rate of a fluid, like blood, and which can be used in conjunction with other instruments or equipment (such as optical sensors or balloon catheters).